Digital potentiometers, like their mechanical counterparts, are resistors whose values can be changed via an external input. The difference is that analog potentiometers are controlled by the rotational position of a control shaft, and the resistance of a digital potentiometer is determined by a digital input presented to the device. Unlike their mechanical, or analog counterparts, digital potentiometers are not continuously variable, but change value only by steps. They are also very limited in the amount of current they can handle, so they are most often used in the feedback loops of operational amplifiers to control the op amp’s gain, and in other similar low-current environments.
The Resistor Ladder
The key element in a digital potentiometer is the resistor ladder, which is a series of resistors, of equal value, connected in series.
Figure 1: Digital potentiometer block diagram. (Source: Analog Devices)
In the aforementioned example, the resistor ladder is composed of seven resistors. If each resistor were 1,000 ohms, the total resistance, Terminal B to Terminal A, would be 7,000 ohms. The adjustable element of the unit is the wiper.
At each of the six junctions between the seven resistors there is a switch, which connects to the wiper. There are also two more switches to connect the wiper, if chosen, to terminals A or B. It is important to keep in mind that only one of the wiper switches can engage at any one time. Thus, the wiper’s resistance, with respect to Terminal B and Terminal A would be, in this example, adjustable in steps of 1,000 ohms, from 0 to 7,000. Note the “3-To-8 decoder.” An N-bit digital word can be decoded into 2^N choices, and 2^3 is 8. Each of those eight possible numbers uniquely defines which of the eight switches is to be engaged at any one time.
It can readily be seen that by using the two terminals and the wiper to implement a voltage divider, the digital potentiometer can be used as a volume control. By using just the wiper and either terminal A or B, a simple variable resistor is available.
How Does a Digital Potentiometer Work?
Upon powering up, some digital potentiometers will simply assign the wiper’s position to the middle of the ladder, or to some other point defined in the datasheet. More often, the device will include enough EEPROM to remember the unit’s last position at the time of the previous power-down. The actual switches that connect the resistors to the wiper are generally implemented in CMOS within the device, as illustrated in the diagram.
Of course, different digital potentiometers can be controlled in a variety of ways. The well-known Serial Peripheral Interface (SPI) bus is a popular choice, as is the simple I2C serial bus. The latter requires only two inputs, one for clock and one for data.
Pins of a Potentiometer
The Analog Devices AD5160BRJZ50 is an 8-pin digital potentiometer that fits into a tiny 2.9 mm x 1.6 mm package. In order to save pin count and real estate, this device doesn’t include EEPROM and employs the SPI bus and not a parallel digital interface. The datasheet reveals the AD5160 family of devices. All members of this group include a resistor ladder composed of 255 separate resistors.
Figure 2: The AD5160 from Analog Devices. (Source: Analog Devices)
The chip is selected via pin 6. The clock enters via pin 4, and the data that selects the wiper position enters on pin 5. Terminals A and B are accessed on pins 7 and 8, and the wiper on pin 1.
Tiny mechanical devices can be fashioned to create electrical pulses that can be used to control a digital potentiometer. This allows them to be controlled by humans, and not only via microcontrollers. They are often fitted into devices only barely bigger than an old-fashioned mechanical potentiometer would be just by itself.